Cleavage of the relatively inert dinitrogen (N.sub.2) molecule, with its extremely strong N.tbd.N triple bond, has represented a major challenge to the development of N.sub.2 chemistry. The relatively inert dinitrogen molecule (N.sub.2) composes 78% of the Earth's atmosphere; the development of this molecule's chemistry is clearly desirable if this immense natural resource is to be utilized optimally. In this regard, the discovery of mild methods for scission of the N.tbd.N triple bond represents a major challenge.
The Haber-Bosch ammonia synthesis is the premier example of industrial nitrogen fixation. This process reacts hydrogen and nitrogen at high temperatures and pressures, in the presence of an iron catalyst to produce ammonia, according to eq. (1), ##STR1## Based on recent ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy studies, the following mechanism has been proposed for the formation of ammonia, whereby the nitrogen can be adsorbed on an iron surface in both the atomic and molecular states. EQU H.sub.2 =2H.sub.(ad) EQU N.sub.2 =N.sub.2(ad)(.gamma.)=N.sub.2(ad)(.alpha.)= 2N.sub.(ad) EQU N.sub.(ad) +H.sub.(ad) =NH.sub.(ad) EQU NH.sub.(ad) +H.sub.(ad) =NH.sub.2(ad) EQU NH.sub.2(ad) +H.sub.(ad) =NH.sub.3(ad) =NH.sub.3
Unfortunately, typical operating temperatures and pressures are in the range of 400-550.degree. C. and 100-1000 atm, respectively, thus rendering this process extremely dangerous. Additionally, the necessary equipment for this process is very large and expensive. Naturally, the development of processes at lower temperatures and pressures, preferably standard pressure and temperature, would be economically very attractive, and would reduce the danger involved.
Several modifications of the Haber-Bosch process, such as the Kellogg Ammonia Process, the Topsoe Ammonia Process, the ICI AMV Ammonia Process, and the Braun Purifier Process, have attempted to address these concerns, and have succeeded in increasing efficiency while modestly lowering the temperatures and pressures required (350-470.degree. C., 70-105 bar). (C. Hooper, in Catalytic Ammonia Synthesis, J. R. Jennings, Ed. Plenum, New York, 1991). However, these processes still operate at very high temperatures and pressures and the equipment involved is still very specialized, large, and expensive. Thus, there is a continuing interest in developing a catalyst system that would operate at standard temperature and pressure.
The metalloenzyme nitrogenase constitutes a unique biological nitrogen-fixing system capable of nitrogen triple bond cleavage at ambient temperatures and pressures. Nitrogenase catalyzes the reduction of molecular nitrogen to ammonia together with the production of dihydrogen under mild conditions, according to eq. (2), EQU N.sub.2 +8H.sup.+ +8e.sup.- .fwdarw.2NH.sub.3 +H.sub.2 (2)
For many years effort has been expounded in an attempt to develop a model system for this unique biological system. The mechanism of binding and reduction in the biological system has remained elusive, however, recently the crystal structure of the active site in nitrogenase was solved. It is believed that the substrate binding and reduction occur at the multimetallic site involved in the FeMo protein, which consists of Mo and Fe atoms bridged by sulfide ligands. (M. K. Chan et al., Science, 260, p. 792 (1993)). Additionally, nitrogenases have also been discovered which contain vanadium in place of molybdenum or only iron as the transition metal component. This suggests that a wide range of transition metals could potentially facilitate reactions of nitrogen in the coordination sphere. Towards this end, studies of the synthesis and reactions of N.sub.2 complexes have been of particular interest in this field. In particular, this area emerged as a result of the discovery in 1965 by Allen and Senoff that [Ru(NH.sub.3).sub.5 ].sup.2+ could reversibly coordinate dinitrogen. (A.D. Allen and C. V. Senoff, J. Chem. Soc., Chem. Commun., p. 621, (1965)).
Since the initial discovery of a complex that could reversibly coordinate dinitrogen, a plethora of N.sub.2 metal complexes have been isolated and characterized. N.sub.2 is able to bond to a variety of metals with a variety of co-ligands. The nature of the bonding in these complexes varies, from end-on bonding in which the N.tbd.N bond distance is similar to that in gaseous N.sub.2 to linear end-on and side bridging to two or more metals. See, George et al. in "Modeling the N.tbd.N Bond-Cleavage Step in the Reduction of Molecular Nitrogen to Ammonia", Molybdenum Enzymes, Cofactors, and Model Systems, Ch. 23, pp. 363-376 (1993).
Unfortunately, well-characterized synthetic systems capable of splitting N.sub.2 have been elusive despite the multitude of known transition-metal complexes containing intact dinitrogen as a ligand. George et al. in "Reduction of Dinitrogen to Ammonia and Hydrazine in Iron(O) and Molybdenum(O) Complexes Containing the N(CH.sub.2 CH.sub.2 PPh.sub.2).sub.3 Ligand" (Inorg. Chem. 34:1295-1298 (1995)) describes the reactions of Fe(N.sub.2)(NP.sub.3) and Mo(N.sub.2).sub.2 (NP.sub.3) with HBr, where NP.sub.3 is N(CH.sub.2 CH.sub.2 PPh.sub.2).sub.3. Very low yields of hydrazine (N.sub.2 H.sub.4) and N.sub.2 were reported.
In all these complexes, there is no demonstrable activation of the N.tbd.N triple bond. Further, the coordination number of the complexing metal is rather high and in all cases is greater than three, indicating that the metal is not in a very activated state. It is therefore desirable to develop a system having an activated nitrogen triple bond to permit product formation under mild conditions.
It is an object of the present invention to provide a process by which soluble, homogeneous metal complexes are capable of catalyzing the formation of ammonia at ambient temperatures and pressures. It is a further object of the present invention to provide a metal complex possessing an activated nitrogen triple bond which can readily undergo reaction with additional reagents. It is a further object of the invention to provide a metal complex capable of activating a nitrogen-nitrogen triple bond. It is a further object of the invention to provide a metal complex capable of activating a variety of small molecules for reaction with additional reagents.